1. Field of Application
The invention concerns a gas sensor comprising a gas-sensitive semiconductor, as well as a gas detection system using such a sensor.
2. State of the Art
A publication entitled "Figaro Gas Sensor TGS 812" describes several embodiments of a gas-sensitive semiconductor of the TGS (Taguchi Gas-sensing Semiconductor) type as well as a number of possible applications of said semiconductor in a gas detector system. The system is based on the principal that the electric resistance in TGS devices increases in oxidizing gases and decreases in reducing gases. In addition, the electric resistance depends also on the temperature and the relative humidity of the atmosphere in which the TGS device is placed, as well as on variations in voltage V.sub.h or V.sub.c, more specifically the voltage that heats the device, or its output voltage. Aiming at a simple and effective elimination of unwanted effects, caused by variations in the ambient temperature and humidity, the device must be adjusted to a working level coinciding with a value taken from a stable part of the temperature and humidity curve. However, this method creates a disadvantage in that the measuring sensitivity of the semiconductor (i.e. the change of resistance as a function of the gas concentration) will be decreased. This means that if a gas detection system employs a single TGS device, which is usual, the requirements for simplicity of structure, chemical reliability, concerning both measuring sensitivity and selectivity, as well as immunity to disturbing variations, can only be met problematically. Moreover, the techniques presently known cannot offer a solution to the problem involved in realizing a detection system with a basically non-selective TGS device, which will be immune to effects caused by chemical compounds normally occurring in the atmosphere.
It is an object of the invention to provide a solution to the problems outlined above. More specifically the aim of the invention is to provide a gas sensor or gas detection system which meets the following combination of requirements:
selective detection with regard to definable groups of gases;
a high measuring sensitivity, which means that the system must be capable of detecting small concentrations of one of the preselected gases;
simplicity of embodiment;
immunity to variations in ambient and operating parameters; and
a low cost price.